Adenosine is present in all cells, including neurons and glia, of mammalian organisms where it modulates a variety of important physiological processes. The action of adenosine is mediated by specific receptors, which belong to the family of G protein-coupled receptors. Four adenosine receptors have been cloned and characterized, A1, A2A, A2B and A3 (Fredholm et al, 1994, Pharmac. Rev., 46, 143-156). The main intracellular signaling pathways involve the formation of cAMP, with A1 and A3 receptors causing inhibition of adenylate cyclase and A2A and A2B receptors activating it (Olah et al, Pharacol. Ther., 2000, 85, 55-75).
All of the adenosine receptors have been located in the CNS (Impagnatiell O et al, Emerg. Ther. Targets, 2000, 4, 635-644; Rosin et al, J. Comp. Neurol., 1998, 401, 163-186). The receptor of interest here, A2A, is predominantly found in dopamine-rich areas, such as the basal ganglia components; the striatum and the globus pallidus, in various mammalians, including humans. The basal ganglia, with the striatum as a central component, are involved in integration of cortical, thalamic and limbic information to produce motor behaviours (for review see Svenningson et al, Prog. Neurobiol., 1999, 59, 355-396).
In the striatum A2A and dopamine D2 receptors are found closely co-localized on the striatopallidal GABAergic neurons, forming the so-called indirect output pathway from the striatum, which is involved in motor inhibition. A2A receptors contribute to control of motor behaviour by modulating the neurotransmission of GABA, dopamine, acetylcholine and glutamate in various ways. Currently, the interactions between A2A and D2 receptors, and especially the actions of A2A antagonists, is of great interest in the treatment for Parkinson's disease (PD). The A2A receptors interact tonically and antagonistically with the D2 receptors, causing a decrease in affinity of the D2 receptors for dopamine upon stimulation. Thus, A2A antagonists may be capable of enhancing the effect of endogenous dopamine as well as clinically used dopamine agonists and increase the time-period of dopaminergic drug response. (For details and references therein see e.g: Richardson et al, Trends Pharmacol. Sci., 1997, 18, 338-344; Svenningson et al, Prog. Neurobiol., 1999, 59, 355-396; Fuxe et al, Parkinson's Dis. Adv., 2001, 86, 345-353).
Selective A2A receptor agonists and antagonists have been widely described in pharmacological, behavioural and neuroprotective experiments in rodents and non-human primates (for reviews see: Richardson et al, Trends Pharmacol. Sci., 1997, 18, 338-344; Ribeiro et al, Prog. Neurobiol., 2003, 68, 377-392; Ongini et al, Il Farmaco, 2001, 56, 87-90; Wardas, Polish J Pharmacology, 2003, 54, 313-326).
The close interaction of D2 and A2A receptors can be clearly exemplified in models of catalepsy, where D2 receptor antagonists as well as A2A receptor agonists induce catalepsy, which is counteracted by A2A receptor antagonists and D2 receptor agonists, respectively (see Svenningson et al, Prog. Neurobiol., 1999, 59, 355-396 and references therein).
Promising anti-parkinsonian effects of A2A receptor antagonists have currently been reported by many investigators. For example, both SCH58261 (2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine) and KW-6002 (8-[(1E)-2-(3,4-dimethoxyphenyl)ethenyl]-1,3-diethyl-3,7-dihydro-7-methyl-1H-purine-2,6-dione), enhance contralateral rotations, elicited by a subtreshold dose of levodopa, in unilateral 6-OHDA (6-hydroxydopamine) lesioned mice and rats (See Ongini et al, Drug Dev. Res., 2001, 52, 379-386 and references therein). Furthermore, KW-6002 significantly improves motor impairment induced in non-human primates by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), without causing dyskinesias, that is commonly described for long-term treatment with the dopamine agonist L-dopa (Kanda et al, Ann. Neurol., 1998, 43, 507-513; Grondin et al, Neurology, 1999, 52, 1673-1677; Kanda et al, Exp. Neurol., 2000, 162, 321-327).
Thus, A2A receptor antagonists show great potential as future drugs for long-term medication of PD patients, since they do not only reverse the motor impairment but also can slow down or stop the progress of the disease by promoting cell survival.
Neuroprotective effects by A2A receptor antagonists have recently been reported in in vivo and in vitro models of different neurodegenerative diseases (for review see: Wardas J., Pol. J. Pharmacol. 2002, 54, 313-26 and Stone T W. Adv. Exp. Med. Biol. 2002, 513, 249-80). A2A antagonists have been shown to be neuroprotective in different PD models like in MPTP treated mice and 6-OHDA-lesioned rats. Here, KW-6002 prevented functional loss of dopaminergic nerve terminals in the striatum as well as prevented gliosis normally induced around degenerating neurons (Ikeda et al, J. Neurochem., 2002, 80, 262-270; Hirsch et al, Adv. Neurol., 1999, 80, 9-18; Kanda et al, Ann. Neurology, 2000, 43 (4), 507-513, Lundblad et al. J. Neurochem. 2003, 84(6), 1398-410). Similar results have been obtained in experimental models of Huntington's disease (HD). In rat HD models quinolinic acid or kainate induced lesions were reduced after using adenosine A2A receptor antagonists, with a decrease in striatal cell loss and motor changes (Reggio et al, Brain Res. 1999, 831, 315-318; Popoli et al, J. Neurosci., 2002, 22, 1967-1975). In addition, it has been shown that A2A receptor antagonists decrease neuronal cell death after cerebral ischemia in neonatal and adult rats and gerbils (Gao Y, Phillis J W., Life Sci. 1994, 55(3), PL61-5; Monopoli A. et al, Neuroreport, 1998, 9(17), 3955-9). A2A knock out animals have been reported to be protected from neonatal hypoxic ischemia and transient focal ischemia (Bona E. et al, Neuropharmacology, 1997, 36(9), 1327-38; Chen J F. et al, J Neurosci, 1999, 19(21), 9192-9200) and from 3NP (3-nitropropionic acid) induced, presynaptic, neurotoxic glutamate release (Blum D. et al, J. Neurosci, 2003, 23, 5361-5369). The protective effect of A2A antagonists against neurodegeneration by glutamate release have allready been shown in a rat model of ischemic damage to the cerebral cortex (Simpson R E, J Neurochem, 1992, 58, 1683-1690 and O'Regan M H. et al, Brain Res, 1992, 582, 22-26).
Protection by A2A antagonists has also been reported in primary astrocytes, in a rat model of bFGF induced astrogliosis, an amyloid beta peptide 25-35 induced neurotoxicity in cerebral granule cells (CGCs) and model of QA induced neuronal cell death in rat organotypic slice cultures (Brambilla R. et al. Glia. 2003, 43, 190-194; Dall'Igna O P. et al. Br. J. Pharmacol. 2003, 138:1207-1209; Tebano M T, et al. gEur. J. Pharmacol. 2002, 253-257)
Collectively, A2A receptor antagonists can efficiently protect different neurons from various forms of insult induced neurodegeneration (Abbracchio M P, Cattabeni F 1999 Ann. NY Acad. Sci. 890: 79-92; Ongini E. et al, Ann. NY Acad. Sci., 1997, 825: 30-48).
Adenosine and its analogues induce “depressant-like” effects in animal models of psychiatric disorders (Minor et al., Behav. Neurosci., 1994, 108: 265-276; Woodson et al., Behav. Neurosci. 1998, 112: 399-409). Moreover, these behavioural deficits were found to be reversed by adenosine A2A receptor antagonists (Minor et al., Behav. Brain Res. 2001, 120, 230-212). Further studies have shown that treatment with adenosine or 2-chloroadenosine increased immobility time in the mouse forced swimming test, another animal model of depression generally considered reliable (Porsolt et al., Arch. Int. Pharmacodyn. Ther., 1977, 229: 327-336).
Several compounds with dual affinity for A2A and A1 receptor subtypes, known as the 4-amino[1,2,3]triazolo[4,3-a]quinoxalines, has been shown to be active in the rat forced swimming test (Sarges et al., J. Med. Chem., 1990, 33, 2240-2254) indicating antidepressant activity of the substances. Most recently, A2A receptor knockout mice were found to be less sensitive to “depressant” challenges than their wildtype littermates (El Yacoubi et al., Br. J. Pharmacol. 2001, 134, 68-77). Consistent with this data, the A2A receptor antagonists SCH58261 and KW6002 reduced the total immobility time in the mouse tail suspension test (El Yacoubi et al., Br. J. Pharmacol. 2001, 134, 69-77). The antagonists SCH58261 and ZM241385 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]-ethyl)phenol were also found to reduce immobility when administered to mice previously screened for having high immobility timne, while SCH58261 reduced immobility of mice that were selectively bred for their “helplessness” in this model (El Yacoubi et al., Br. J. Pharmacol. 2001, 134, 68-77).
Studies using A2A knockout mice suggest that these animals show a blunted response to psychostirnulants such as amphetamine and cocaine, despite the fact that their expression and binding affinities of D1 and D2 receptors are unaffected (Chen et al., Neurosci., 2000, 97, 195-204). Moreover, inactivation of A2A receptors has been shown to selectively attenuate amphetamine-induced behavioural sensitisation (Chen et al., Neuropsychopharmacol., 2003, 28, 1086-1095). In addition, A2A knockout mice show reduced startle and PPI of the acoustic startle (Wang et al., Behav. Brain Res., 2003, 143, 201-207), measures often used to detect antipsychotic activity. Further support is found in studies where pharmacological blockade of A2A receptors with a selective antagonist completely abolished pre-pulse imhibition (PPI) (Nagel et al., Synapse, 2003, 49, 279-286). Psychostimulants, such as MK-801 and amphetamine failed to disr-upt startle and PPI in A2A KO mice (Wang et al., Behav. Brain Res., 2003, 143, 201-207).
Thus, the available evidence suggests that adenosine A2A receptor antagonists, by specifically modulating mesostriatal or mesocorticolimbic dopaminergic pathways, may possess antidepressant and/or antipsychotic properties
WO02/42298 discloses compounds of the formula:
as A2B receptor antagonists which in general selectively inhibit activation of the A2b receptor over the adenosine A1 and A2A receptors. The compounds are disclosed as being useful in the treatment of inflammatory or obstructive airways diseases.
Hence, there is a desire for novel A2A-receptor ligands, such as antagonists, agonists, reverse agonists or partial agonists.